When blood vessels are damaged, cell fragments released from the bone marrow, called platelets, adhere to the walls of blood vessels and form clots to prevent blood loss. It is important to have adequate numbers of normally functioning platelets to maintain effective clotting, or coagulation, of the blood. Occasionally, when the body undergoes trauma, or when the platelets are unable to function properly, it is necessary to replace or transfer platelet components of blood into a patient. Most commonly, platelets are obtained from volunteer donors either as a component of a whole blood unit, or via plateletpheresis (withdrawing only platelets from a donor and re-infusing the remaining of the blood back into the donor). The platelets then are transferred to a patient as needed, in a process referred to as “platelet transfusion”.
Platelet transfusion is indicated under several different scenarios. For example, an acute blood loss, either during an operation or as a result of trauma, can cause the loss of a large amount of platelets in a short period of time. Platelet transfusion is necessary to restore a normal ability to control blood flow, or haemostasis. In a medical setting, an individual can develop a condition of decreased number of platelets, a condition known as thrombocytopenia. The condition can occur as a result of chemotherapy, and requires platelet transfusion to restore normal blood clotting.
Unlike red blood cells, which can be stored for forty-five (45) days, platelets can be stored for only five to seven days. The short storage term, or shelf-life, of the platelets severely limits the useful span for a platelet supply. A consequence of this short shelf-life is that platelets must be collected close to their time of use, which makes it extremely difficult to coordinate platelet collection and platelet supply.
One reason that platelets have such a short shelf-life is because platelets become activated during the process of collection. The activation process leads to externalization of platelet canalicular surfaces exposing receptor sites, such as GPIIb/IIIa. Phosphatidylserine residues on activated platelets tend to cause platelet aggregation, which results in cell death (i.e., apoptosis) upon re-infusion into patients. Thus, a platelet's functional half-life is significantly reduced.
Another reason that platelets have a short shelf-life is because an inadequate oxygen supply alters the metabolic activity of the platelets. In an environment lacking a sufficient oxygen supply, the platelets undergo an anaerobic mechanism leading to accumulation of lactic acid. The increased concentration of lactic acid causes a drop in pH, and results in cell death. Although platelets can be stored in gas permeable bags using a shaker bath under a stream of air to help overcome this problem, such storage methods are costly and extremely inefficient and inadequate in meeting the oxygen requirements of the stored platelets.
Platelet sterility is difficult to maintain because platelets cannot be stored at low temperatures, for example −80° C. to −40° C. As previously mentioned, a low storage temperature for the platelets initiates an activation process within the platelets that leads to aggregation and cell death. Nonetheless, bacterial growth in the platelet medium at suitable storage temperatures, e.g., room temperature, can lead to an unacceptable occurrence of bacterial contamination in platelets used for transfusion. As a result, the Food and Drug Administration (FDA) limits the storage time of platelets to five days, thereby safeguarding the transfusion supply from bacterial contamination.